Technical Field
The present invention relates to a control apparatus for a power supply system including a first rechargeable battery, such as a lead-acid battery, and a second rechargeable battery, such as a lithium-ion battery, having a higher output density or energy density than the first rechargeable battery.
Related Art
A lead-acid battery (as a first rechargeable battery) is commonly mounted in a vehicle driven by an internal-combustion engine to supply electrical power to various electrical loads, such as a starter motor. The first rechargeable battery is cheaper than a second rechargeable battery that is a high power and high energy density battery, such as a nickel battery or a lithium-ion battery, but less durable for frequent charging and discharging. In particular, as a concern, vehicles provided with a stop and start function suffer from premature degradation of the first rechargeable battery caused by frequent discharging of the first rechargeable battery. As another concern, vehicles having an alternator used to generate charging power from vehicle's regenerative energy also suffer from premature degradation of the first rechargeable battery caused by frequent charging of the first rechargeable battery.
The second rechargeable battery is more expensive than the first rechargeable battery. Therefore, simply replacing the first rechargeable battery with the second rechargeable battery will lead to a significant increase in cost.
Conventionally, a technique has been proposed such that the second rechargeable, battery that is durable against frequent charging and discharging and the first rechargeable battery that is inexpensive are electrically connected in parallel to be mounted in the vehicles. In such a technique, when idling is stopped, the second rechargeable battery is preferentially used to supply power to electrical loads and preferentially charged, thereby mitigating degradation of the first rechargeable battery. In addition, when the vehicle is parked, the first rechargeable battery is preferentially used for power supply required over a long time period, whereby the increase in cost is prevented by reducing the capacity of the second rechargeable battery.
Upper and lower limits are defined for a state of charge (SOC) of the second rechargeable battery. The SOC is a ratio of an actual amount of charge to an amount of charge when fully charged. If the SOC is above the upper limit or if the SOC is below the lower limit, premature degradation may occur. To prevent such premature degradation, if the lower limit is reached when idling is stopped, the internal-combustion engine has to be started to charge the second rechargeable battery by using the alternator, which will lead to deterioration in fuel efficiency. If a target SOC that is a target value of SOC of the second rechargeable battery is set to the upper-limit to prevent the SOC from reaching the lower limit when idling is stopped, chances for power regeneration may be lost if the SOC has reached the upper limit when the regenerative power generation becomes available.
A control apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2012-90404 can solve such a problem. This control apparatus is configured to; based on power consumed when idling of an internal-combustion engine is stopped, modify a target SOC so that the SOC does not decrease below the target SOC. With such a configuration, the opportunity for charging with regenerative power can be ensured.
Regarding processing to modify the target SOC, as in the control apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2012-90404, the SOC may be less than the modified target SOC at the modification of the target SOC. In such an event, the alternator will be operated to increase the SOC to the modified target SOC. Thus, the internal-combustion engine will be loaded, which may reduce the rotational speed of the internal-combustion engine. This may cause the driver of the vehicle to sense an unexpected braking force, which may result in deterioration of drivability.
In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing a control apparatus capable of preventing deterioration of drivability.